The magnetic disk is one of the typical information recording medium having a recording layer utilizing properties of magnetism, light, and magnetooptics. Aluminum substrates have been widely used as a magnetic disk substrate. However, in recent years, there has been an demand for reduction in the levitation height of a magnetic head for improvement in the recording density. To meet this demand, there has been an increasing percentage in the use of the glass substrate as a magnetic disk substrate, because the glass substrate can reduce the levitation height owing to a smoother surface and fewer surface defects than the aluminum substrate.
In the method for manufacturing the aforementioned glass substrate for an information recording medium such as a magnetic disk, it is a common practice to reinforce the glass substrate by providing chemical reinforcing treatment on the surface of the glass substrate for the purpose of enhancing crashworthiness and vibration resistance of a glass substrate to prevent the substrate from being damaged by impact or vibration. Chemical reinforcing treatment is normally performed by an ion exchange method where a glass substrate is dipped in a chemical reinforcing solution, and the Na+ ion on the surface of the glass substrate is exchanged with the K+ ion in a chemical reinforcing solution, whereby a compressive stress layer is formed on the surface of the glass substrate.
After chemical reinforcement, the chemically reinforced surface is usually subjected to a polishing process in order to remove erosion by the chemical reinforcing solution and fine scratches thereby ensuring the required smoothness. However, there is a problem that the glass substrate warps and the flatness of the substrate is degraded due to the imbalance between the stresses of the frond and back surfaces because a part of the compressive stress layer on the glass substrate is removed in the polishing process.
In order to prevent the degradation of the flatness, proposed is an information recording glass substrate where the reduction in the thickness of the glass substrate by polishing after the chemical reinforcement is controlled in the range of 0 μm or more without exceeding 0.4 μm on each side (Patent Document 1).
Patent Document 1: Japanese Unexamined Patent Application Publication No. H11-268932.